Rich tool UI restored and ToolFallback exported

This release restores the default rich tool-call rendering in the playground and exposes ToolFallback from the public API so consumers can compose around it.

Features

• Restore rich tool UI in tool calls #2221  - The MCP Apps integration had replaced the default tool UI for every tool call. Tool calls now delegate to the default ToolFallback, and the MCP App iframe is only appended when a tool has a UI resource binding. Elements now exports ToolFallback from its public API. (Author: @simplesagar )